Conventional two stroke gasoline engines in current use, as in the outboard motor and lawnmower market, are extremely simple employing only three moving parts per cylinder, but lack efficiency as compared to four stroke engines. This is primarily due to scavenging the cylinder with a mixture of air and fuel with considerable loss of unused fuel through the exhaust.
Partial scavenging with compressed air before fuel injection has been suggested in a two cycle internal combustion engine disclosed in U.S. Pat. No. 2,425,157. Fuel is injected into a continuously air pressurized intake port uncovered by the piston near the bottom of its stroke. The exhaust port is valve controlled. A cam is timed so that it opens the fuel nozzle and permits fuel to be admitted into the fuel pipe and cylinder slightly later than the opening of the intake and exhaust ports. Admittance of the fuel will be completed slightly before the intake port is closed on the up stroke of the piston.
Another approach to scavenging with compressed air prior to fuel injection has been disclosed in U.S. Pat. No. 2,887,993 wherein pressurized air is introduced through a plurality of small circumferentially spaced ports in the cylinder wall uncovered by the piston near the bottom of its stroke with a valve controlled exhaust port. Such patent discloses a spark ignition engine wherein a full charge of air is drawn in during an air intake period and spill metering is applied to get rid of whatever portion of the air cannot be used. This is accomplished by retarding the closing of the exhaust valve during the compression stroke. The quantity of fuel injected into the cylinder is controlled by a fuel injection pump and the engine enables both air and fuel to be decreased as the load is decreased in order to maintain an efficient mixture ratio and reduce pumping losses. Intake air is moved into the cylinder and the excess air is moved out of the cylinder during the compression stroke through the exhaust valve and after excess air is moved out of the cylinder, the exhaust valve is closed, fuel is injected into the cylinder, the spark plug ignites the mixture and the piston is moved in the opposite direction. Thereafter the exhaust valve opens resulting in the discharge of the exhaust gases.